The entire disclosure of Japanese Patent Application No. 2000-203530 filed on Jul. 5, 2000 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a reduced iron discharger in a rotary hearth reducing furnace for producing reduced iron by reducing, in a high temperature atmosphere, pellet-or briquette-like agglomerates which have been formed from a powdery mixture of an iron oxide powder and a reducing agent and supplied onto a rotary hearth.
2. Description of the Related Art
To produce reduced iron, the first step is, generally, to mix a powder of iron ore (iron oxide), a powder of coal (reducing agent), a powder of limestone (fluxing agent), and a binder such as bentonite, and to compress and pelletize the mixture to form wet balls called xe2x80x9cgreen balls.xe2x80x9d Then, the wet balls are dried to some degree to form dry balls. The dry balls are heated to a high temperature in a reducing furnace, where the iron oxide in the iron ore is reduced by the coal as a reducing agent to form reduced iron in the form of pellets.
An example of an apparatus for producing such reduced iron is explained by way of FIG. 7. Powders of iron ore, coal, etc. and a binder are mixed in a mixer (not shown). The resulting mixed powder is pelletized in a pelletizer 1 to form green balls (green or raw pellets) GB. Then, the green balls GB are charged into a dryer 2, where they are dried with an off-gas from a reducing furnace 4 (to be described later on) to form dry balls DB. The dry balls DB are supplied to the reducing furnace 4 by a pellet feeder 3.
The interior of the reducing furnace 4 is maintained in a high temperature atmosphere upon heating by a burner 5, and an inside off-gas is discharged from an off-gas duct 6. Thus, the dry balls DB are preheated and heated with radiant heat from the wall of the furnace when they are passed through the interior of the reducing furnace 4. During their passage, the iron oxide in the iron ore is reduced with the coal as the reducing agent to form reduced iron in the form of pellets. The reduced pellets are discharged to the outside by a pellet discharger 8, and accommodated into a portable vessel 9.
The off-gas from the off-gas duct 6 usually contains some unburned gas, and is thus burned in an after burner chamber 7 nearly completely. Then, the off-gas is cooled in a water spray primary cooler 10, and then sent to a heat exchanger 11, where it undergoes heat exchange. Combustion air heated by the heat exchange is sent to the reducing furnace 4, and fed into the furnace together with fuel. On the other hand, the off-gas is cooled again in a secondary cooler 12, and part of it is sent to the dryer 2 as drying air for the green balls GB as stated earlier. The remaining part of the off-gas is cleaned in a dust collector 13, and released into the atmosphere via a stack 14.
A screw discharger as shown in FIG. 8 has been used as the pellet discharger 8. When this discharger is used, a rotary hearth 15 is supported by a floor rail 16 disposed concentrically in a furnace chamber, and a horizontal roller 18 disposed in an inner peripheral portion of a furnace wall 17 in such a manner that a wheel 19 contacts the floor rail 16 and a side surface rail 20 of the rotary hearth 15 itself contacts the horizontal roller 18. The rotary hearth 15 is rotated by a rotational drive system (not shown), with a space between the rotary hearth 15 and the furnace wall 17 being sealed with a water groove 21. A discharge screw 62 having a spiral blade 62a is mounted across the rotary hearth 15, with a tiny gap being kept between the discharge screw 62 and the upper surface of the rotary hearth 15, and a shaft end portion of the discharge screw 62 is supported by a bearing 63. The discharge screw 62 is rotated by a motor 64 in the direction indicated by an arrow 65 in the drawing. As a result, reduced iron P on the rotary hearth 15 is raked out by the spiral blade 62a toward a discharge port on the right side in the drawing.
With the conventional screw discharger, the reduced iron raked out from a site on the moving rotary hearth 15 in a perpendicularly lateral direction by the spiral blade 62a increases in amount and becomes bulky as it approaches the discharge port in the end portion of the discharge screw 62, as shown by the symbol P in FIG. 8. Thus, the height of the spiral blade 62a needs to be consistent with the amount of reduced iron at the discharge port. Hence, the entrance side of the discharge screw 62 (i.e., the side opposite to the discharge port), where the amount of reduced iron is small, faces the problem that the height of the blade made of an expensive heat resistant steel is useless. Besides, during raking-out by the discharge screw 62, the reduced iron at a high temperature is converted into a powder or powdered under the pressure of the spiral blade 62a , resulting in a decreased yield.
The rotational speed of the discharge screw 62 is linked to the volume of production by the reducing furnace. That is, if the discharge screw 62 rotates in the same manner when the amount of green pellets supplied into the furnace increases, not all of the reduced iron P will be discharged, and some of the reduced iron P escapes the discharge screw 62. To increase the volume of production, therefore, the rotational speed of the discharge screw 62 must be increased.
FIG. 9 is a graph showing the relationship between the necessary rotational speed of the discharge screw 62, the rotational speed of the rotary hearth 15, and the volume of production. The horizontal axis represents the volume of production (t/hr), and the vertical axis represents the screw speed (r.p.m.). As an example, the graph shows the course of the necessary rotational speed of the discharge screw 62 in response to changes in volume of production in the reducing furnace whose hearth rotational speed is 6 rotations per hour. When the hearth rotational speed is 6 rotations per hour, the corresponding screw speed is 7 rotations per minute. At this screw rotational speed, the volume producible without escape of reduced iron is up to about 45 tons per hour. To produce a greater volume, the screw rotational speed should be increased in proportion to the increase in the volume of production. When the rotational speed of the discharge screw 62 increases, the speed of the reduced iron P discharged from the reducing furnace becomes high. As a result, powdering of the high temperature reduced iron due to collision is accelerated, aggravating the aforementioned decrease in the yield.
The present invention has been proposed in light of these circumstances. It is an object of this invention to provide a reduced iron discharger in a rotary hearth reducing furnace, which involves minimal structural waste and obtains a satisfactory yield.
A first aspect of the present invention, as a means of attaining the above object, is a reduced iron discharger in a rotary hearth reducing furnace for producing reduced iron by reducing agglomerates in a high temperature atmosphere, the agglomerates being pelletized from a powdery mixture of an iron oxide powder and a reducing agent and supplied onto a rotary hearth, wherein rotary blades capable of discharging the reduced iron from a site on the rotary hearth are provided. Thus, the reduced iron discharger can serve as an apparatus which involves minimal structural waste and obtains a satisfactory yield.
A second aspect of the invention is the above-mentioned reduced iron discharger in a rotary hearth reducing furnace, wherein the blades each comprise a body member and a front end member detachably provided on the body member. Thus, when the front end portion of the blade wears, only the front end member can be replaced easily.
A third aspect of the invention is the above reduced iron discharger in a rotary hearth reducing furnace, wherein the body member is reinforced with a rib. Thus, the durability of the blade is increased.
A fourth aspect of the invention is the above reduced iron discharger in a rotary hearth reducing furnace, wherein the blades are composed of an impeller which rotates about an axis extending across the rotary hearth and scoops up the reduced iron, and a transport device for accepting the reduced iron falling at a rotating ascending position of the impeller, and discharging the reduced iron to the outside of the furnace is mounted in the impeller. Thus, the same effect as in the first aspect of the invention can be obtained.
A fifth aspect of the invention is the above reduced iron discharger in a rotary hearth reducing furnace, wherein the transport device is a vibrating conveyor disposed obliquely across the rotary hearth. Thus, reduced iron can be discharged smoothly.
A sixth aspect of the invention is the above reduced iron discharger in a rotary hearth reducing furnace, wherein the blades are composed of raking-out devices which rotate across the rotary hearth to rake out the reduced iron. Thus, the same effect as in the first aspect of the invention is obtained, and the transport device in the fourth aspect of the invention becomes unnecessary.
A seventh aspect of the invention is the above reduced iron discharger in a rotary hearth reducing furnace, wherein the width of each of the blades is set in accordance with the maximum speed of the rotary hearth. Thus, the amount of reduced iron escaping the raking-out devices can be decreased, without the need to increase the rotational speed of the raking-out devices in response to an increase in the volume of production in an operation for production of up to a high volume.
An eighth aspect of the invention is the above reduced iron discharger in a rotary hearth reducing furnace, wherein cooling means is provided for cooling the blades, which have discharged the reduced iron, above the hearth. Thus, the heat load of the blades is reduced to improve the durability of the blades.